Optoelectronic devices, such as lasers for use in optical communication systems, have to meet very stringent requirements. More specifically, the wavelength locking range of a laser is an important parameter to control and stabilize. In certain applications, however, large changes in environmental temperature, or operating current variations, may cause the laser to become unlocked or locked at the wrong wavelength of light. For example, in submarine applications, where lasers are meant to operate in undersea fiber links, the variation in operating temperature may exceed 40° C. Moreover, there is no thermoelectric cooler in the device package to control the laser chip temperature.
One well-known means of stabilizing the locking range involves coupling an external grated waveguide, such as a fiber-Bragg-grating, to a Fabry-Perot (F-P) laser chip at the output facet of the laser. F-P lasers have a broadband low reflectivity (LR) coating on the output facet. This relatively flat reflectivity spectrum allows the laser to operate in the range of wavelengths where the gain is the highest, the so-called chip wavelength. Grated waveguides, such as Fiber-Bragg-gratings, have their own wavelength of maximum reflectivity, the so-called grating wavelength. For example, when a fiber-Bragg-grating is coupled to the output facet of a F-P laser, so long as the chip and the grating wavelengths are substantially similar, the laser can lock and lase at the grating wavelength, instead of the chip wavelength. Under the above mentioned conditions, however, because the gain spectrum of the chip is sensitive to temperature, the chip wavelength may shift significantly away from the grating wavelength. Consequently, instead of oscillating or locking at the grating wavelength, the laser will prefer to lase at the chip wavelength. Under such circumstances the chip laser is said to be outside of the locking range of the grating waveguide.
Previous efforts to resolve this problem have not lead to entirely satisfactory solutions. For example, the locking range of a fiber-Bragg-grating stabilized F-P laser may be increased by increasing the maximum reflectivity of the fiber-Bragg-grating. However, increased grating reflectivity may result in reduced output power. This may be especially severe for longer higher-power laser chips that function optimally with a high output coupling. Alternatively, the necessary locking range may be reduced by specifying a reduced operating temperature for the product. But a reduced operating temperature range may not be attractive to customers because this requires increased inventory management. Finally, a grating internal to the laser chip, such as a diffraction grating, may be used to form a distributed feed back (DFB) laser to facilitate stabilization of the lasing wavelength, instead of an external fiber-Bragg-grating. However, such DFB lasers are unattractive for use in uncooled Raman applications, because such lasers still have a significantly greater temperature dependent shift (i.e., chip wavelength ˜0.09 nm/° C.), as compared to the temperature dependence of a laser coupled to an external grated waveguide (i.e., grating wavelength ˜0.01 nm/° C.).
Accordingly, what is needed in the art is an optoelectronic device having an increased locking range that does not experience the drawbacks encountered by the conventional devices and resolutions listed above.